Update build status image to use GitHub Actions

[alexellis]

Expected Behaviour

Correct build status to be shown

Current Behaviour

Shows the old Travis build.

Possible Solution

Update the link as per the README of openfaas/faas. Note that it has both a link to the build and an image for the badge.

[alexellis]

/add label: good first issue, help wanted

[alexellis]

This has been done now.